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TRS-80 Model 4P RAM question

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rlauzon

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Feb 18, 2018
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Grand Rapids, MI
So I'm finally getting around to working on my model 4P.

According to what I've been reading, the issue that I have is that some of the RAM is bad.

The chips are labelled: 8040665 FQD8446, but 2 are 8040665 TQD8446

My question is: Are those really the same? If I picked up some TQD8446 chips, would those work in the FQD8446 slots?
 
rlauzon said:
The chips are labelled: 8040665 FQD8446, but 2 are 8040665 TQD8446

My question is: Are those really the same? If I picked up some TQD8446 chips, would those work in the FQD8446 slots?
Click to expand...

You're unlikely to find any "TQD8446" chips. The "8446" refers to a single production week (week 46 of 1984). The difference between "TQD" and "FQD" is trivial.

Any chips with the Tandy part number on them (8040665) are guaranteed to be compatible. But you don't have to try to find those. Lots of other 4164 chips should work. The main Model 4/4P requirement that not all chips will meet is that they must require only 128 refresh addresses (rather than 256). Here's a good list of lots of manufacturers' 4164 chips and their properties:

http://www.minuszerodegrees.net/memory/4164.htm
 
Last edited:
Just bought a load from Bulgaria, all MN4164P, 150ns with the 128 bit refresh that seems to be the real problem

They are on Ebay at £1.42 for two and while it states not tested, every one I got worked fine (i did buy 16 just to ensure I got enough to replace the bad chip and do the upgrade on my model 4 GA)

serch for MN4164P-15-Page-Mode-DRAM-64KX1-150ns-NMOS-USED-2-pieces on Ebay
 
Thanks!

I was able to source some 4164 chips from Jameco. My 4P only has 64K right now, but it looks like I'll just upgrade it to 128K while I'm fixing the RAM.
 
rlauzon said:
Thanks!

I was able to source some 4164 chips from Jameco. My 4P only has 64K right now, but it looks like I'll just upgrade it to 128K while I'm fixing the RAM.
Click to expand...

Are they the 128 cycle refresh chips or the 256 cycle refresh. If its the latter, they wont work but have similar part numbers.

info here http://www.vcfed.org/forum/showthread.php?36974-4164-DRAM-with-128-refresh

Looking at the Jameco website, https://www.jameco.com/z/4164-120-S...ynamic-RAM-with-Page-Mode-DIP-16_2290535.html type KN4166B have a 128 refresh cycle but https://www.jameco.com/z/4164-150-Major-Brands-IC-4164-150-64Kx1-DRAM-150NS-16-pin-DIP_41662.html do not.
 
Gary C said:
Are they the 128 cycle refresh chips or the 256 cycle refresh. If its the latter, they wont work but have similar part numbers.

info here http://www.vcfed.org/forum/showthread.php?36974-4164-DRAM-with-128-refresh

Looking at the Jameco website, https://www.jameco.com/z/4164-120-S...ynamic-RAM-with-Page-Mode-DIP-16_2290535.html type KN4166B have a 128 refresh cycle but https://www.jameco.com/z/4164-150-Major-Brands-IC-4164-150-64Kx1-DRAM-150NS-16-pin-DIP_41662.html do not.
Click to expand...

Thanks for the warning. I was going to get the wrong chips.
 
So I'm finally able to get back to this project (Ya, 6 months later). Thanks to everyone who've helped.

Short update:
+ The new RAM didn't work. I think that there was a problem with the motherboard. Based on some of the stains I found inside, I think that someone spilled something on it in the long past and had it repaired. But the repair wasn't 100%.
+ I was able to locate a "new" motherboard (with more RAM and the high-res graphics option plus FreHD ROM).
+ I also replaced the RIFA caps while I had everything torn apart.

The good news is that (almost) everything is working. But there's a bad RAM chip. When I run the memory check, I get:

Error occurs at: B280
Data should be 99 is 19
Error is on the high side 8 (U77)
Click to expand...

Of course, I didn't bother to run the diagnostics until after I reassembled everything.

So my next task is to take it apart again and validate the jumpers and that all chips are correctly socketed.

I assume that the "Error occurs at" is telling me where in the memory map the problem is. I should be able to look up which bank that is and have a pretty good idea which RAM chip has the problem.
Luckily, I have plenty of RAM chips from the old motherboard to "Easter egg" chips.
 
RAM issue resolved. The memory check showed it was the 8 bit, so I started by just pulling the current chips and replacing them with some RAM chips that I had from a different 4P.

I wasn't sure which end the 8 bit was. So, of course, I pulled the 0 bit chips first. :) But that was quickly fixed.

I did run all the diagnostics before reassembling this time.
 
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